You Make It Happen with Ken Finn

What makes a rubber ball bounce? Break out a couple balloons and find out! Credit: George Retseck

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Key Concepts
Physics
Deflection
Potential Energy
Kinetic Energy

Introduction
Have you ever wondered about the secret behind a ball's bounce? Observing these bounces, however, can be difficult! A bounce often happens too quickly to see precisely what is occurring as the ball hits and deflects off a surface. In this experiment you're going to see how a ball bounces by watching in slow-motion—in real time!

Background
This experiment is all about two forms of energy: potential and kinetic. If something has potential energy, it has energy that's stored. When you lift an object—a water balloon, for example—against the force of gravity that object now has the potential energy equal to the work done to elevate it. When you drop that same object, that potential energy becomes kinetic. Kinetic energy is the energy of motion: the motion of a water balloon falling, of rubber stretching or of rubber springing back to its original shape. Bouncing is an exercise in energy changing forms from potential to kinetic and back to potential.

You're going to see how a ball bounces by watching it in slow motion. That slow bouncing ball is going to be a superstrong water balloon.

Materials

At least four round balloons (suitable for filling with water)

Cooking oil (You only need a little to make superstrong water balloons.)

Water (to fill a water balloon)

A smooth outdoor surface that can get wet (such as a sidewalk or driveway)

Optional: video camera or video-capable device and a helper

Preparation

Apply the cooking oil to the outside of one round balloon. Why do you think this is important?

Insert this greased balloon inside a second round balloon.

Fill the inner balloon with water and then knot both balloons.

Prepare additional "superstrong" water balloons for the activity now, if you like, in case the first one breaks.

Procedure

Prepare to drop your superstrong water balloon onto a smooth surface by lifting it up and holding it out above the surface. What do you expect to see when the balloon is falling? What about when it hits the surface?

Be prepared to keep a close eye on what happens and then drop the water balloon. What did you notice when the ball fell? What happened when it hit the surface? What happened after it hit the surface?

Repeat the activity a few more times and look closely at the bottom of the water balloon as it makes contact with the surface—and when it bounces back up again. Why do you think this is happening?

Extra: If you want an even more detailed view into the dynamics of the bounce, have a helper use a video camera or smartphone to capture the balloon's bounce at surface level. (This might require bending way down!) You can even play that video back in even slower motion to study the movement. Did you notice anything different in the video that you had missed before?

[break]Observations and results
Did the water balloon maintain its round shape as it was falling but then squash when it hit the ground? Did it then spring back into a round shape as it lifted back up off the surface?

When you lift up your water balloon, you are giving it potential energy—"potential" refers to something that could happen. (You have the potential to become a physicist, for example.) If something has potential energy, it has energy that's stored up. When you drop your water balloon, that potential energy becomes kinetic. Kinetic energy is the energy of motion. (A car moving at 100 kilometers per hour has a lot of kinetic energy.)

When the balloon hits the floor and stops, that kinetic energy has to go somewhere. The kinetic energy goes into squashing the balloon flat. When the balloon squashes, the rubber stretches—and stretched-out rubber wants to snap back to its original shape (think: a stretched rubber band). When the balloon’s stretched rubber snaps back, it pushes against the floor and the floor pushes back. As a result, the balloon goes bouncing back up in the air.

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